A parent class for throwable objects intended for flow control.
Instances of ControlThrowable
should not normally be caught.
As a convenience, NonFatal
does not match ControlThrowable
.
import scala.util.control.{Breaks, NonFatal}, Breaks.{break, breakable} breakable { for (v <- values) { try { if (p(v)) break else ??? } catch { case NonFatal(t) => log(t) // can't catch a break } } }
Suppression is disabled, because flow control should not suppress an exceptional condition. Stack traces are also disabled, allowing instances of ControlThrowable
to be safely reused.
Instances of ControlThrowable
should not normally have a cause. Legacy subclasses may set a cause using initCause
.
A trait for exceptions which, for efficiency reasons, do not fill in the stack trace. Stack trace suppression can be disabled on a global basis via a system property wrapper in scala.sys.SystemProperties.
2.8
Since JDK 1.7, a similar effect can be achieved with class Ex extends Throwable(..., writableStackTrace = false)
An object that can be used for the break control abstraction. Example usage:
import Breaks.{break, breakable} breakable { for (...) { if (...) break } }
Classes representing the components of exception handling.
Each class is independently composable.
This class differs from scala.util.Try in that it focuses on composing exception handlers rather than composing behavior. All behavior should be composed first and fed to a Catch object using one of the opt
, either
or withTry
methods. Taken together the classes provide a DSL for composing catch and finally behaviors.
Create a Catch
which handles specified exceptions.
import scala.util.control.Exception._ import java.net._ val s = "http://www.scala-lang.org/" // Some(http://www.scala-lang.org/) val x1: Option[URL] = catching(classOf[MalformedURLException]) opt new URL(s) // Right(http://www.scala-lang.org/) val x2: Either[Throwable,URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) either new URL(s) // Success(http://www.scala-lang.org/) val x3: Try[URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) withTry new URL(s) val defaultUrl = new URL("http://example.com") // URL(http://example.com) because htt/xx throws MalformedURLException val x4: URL = failAsValue(classOf[MalformedURLException])(defaultUrl)(new URL("htt/xx"))
Create a Catch
which logs exceptions using handling
and by
.
def log(t: Throwable): Unit = t.printStackTrace val withThrowableLogging: Catch[Unit] = handling(classOf[MalformedURLException]) by (log) def printUrl(url: String) : Unit = { val con = new URL(url) openConnection() val source = scala.io.Source.fromInputStream(con.getInputStream()) source.getLines.foreach(println) } val badUrl = "htt/xx" // Prints stacktrace, // java.net.MalformedURLException: no protocol: htt/xx // at java.net.URL.<init>(URL.java:586) withThrowableLogging { printUrl(badUrl) } val goodUrl = "http://www.scala-lang.org/" // Prints page content, // <!DOCTYPE html> // <html> withThrowableLogging { printUrl(goodUrl) }
Use unwrapping
to create a Catch
that unwraps exceptions before rethrowing.
class AppException(cause: Throwable) extends RuntimeException(cause) val unwrappingCatch: Catch[Nothing] = unwrapping(classOf[AppException]) def calcResult: Int = throw new AppException(new NullPointerException) // Throws NPE not AppException, // java.lang.NullPointerException // at .calcResult(<console>:17) val result = unwrappingCatch(calcResult)
Use failAsValue
to provide a default when a specified exception is caught.
val inputDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0) val candidatePick = "seven" // scala.io.StdIn.readLine() // Int = 0 val pick = inputDefaulting(candidatePick.toInt)
Compose multiple Catch
s with or
to build a Catch
that provides default values varied by exception.
val formatDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0) val nullDefaulting: Catch[Int] = failAsValue(classOf[NullPointerException])(-1) val otherDefaulting: Catch[Int] = nonFatalCatch withApply(_ => -100) val combinedDefaulting: Catch[Int] = formatDefaulting or nullDefaulting or otherDefaulting def p(s: String): Int = s.length * s.toInt // Int = 0 combinedDefaulting(p("tenty-nine")) // Int = -1 combinedDefaulting(p(null: String)) // Int = -100 combinedDefaulting(throw new IllegalStateException) // Int = 22 combinedDefaulting(p("11"))
Extractor of non-fatal Throwables. Will not match fatal errors like VirtualMachineError
(for example, OutOfMemoryError
and StackOverflowError
, subclasses of VirtualMachineError
), ThreadDeath
, LinkageError
, InterruptedException
, ControlThrowable
.
Note that scala.util.control.ControlThrowable, an internal Throwable, is not matched by NonFatal
(and would therefore be thrown).
For example, all harmless Throwables can be caught by:
try { // dangerous stuff } catch { case NonFatal(e) => log.error(e, "Something not that bad.") // or case e if NonFatal(e) => log.error(e, "Something not that bad.") }
Methods exported by this object implement tail calls via trampolining. Tail calling methods have to return their result using done
or call the next method using tailcall
. Both return a TailRec
object. The result of evaluating a tailcalling function can be retrieved from a Tailrec
value using method result
. Implemented as described in "Stackless Scala with Free Monads" http://blog.higher-order.com/assets/trampolines.pdf
Here's a usage example:
import scala.util.control.TailCalls._ def isEven(xs: List[Int]): TailRec[Boolean] = if (xs.isEmpty) done(true) else tailcall(isOdd(xs.tail)) def isOdd(xs: List[Int]): TailRec[Boolean] = if (xs.isEmpty) done(false) else tailcall(isEven(xs.tail)) isEven((1 to 100000).toList).result def fib(n: Int): TailRec[Int] = if (n < 2) done(n) else for { x <- tailcall(fib(n - 1)) y <- tailcall(fib(n - 2)) } yield (x + y) fib(40).result
© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala/util/control/index.html
A class that can be instantiated for the break control abstraction. Example usage:
Calls to break from one instantiation of
Breaks
will never target breakable objects of some other instantiation.